English professor Kathleen Kelly grew up reading C.S. Lewis and J.R.R. Tolkien, devouring page after page of high fantasy. And her keen interest in their work, from Tolkien’s The Lord of the Rings to Lewis’ The Lion, the Witch, and the Wardrobe, compelled her to focus her scholarship on the ins and outs of medievalism. But even she could not have predicted the unparalleled success of the Harry Potter series, defined by its sweeping impact on both publishing and pop culture.

“I knew Harry Potter would become canonical like the books in Lewis and Tolkien’s oeuvre,” said Kelly, who’s read all seven Potter novels, “but I had no idea it would become such a phenomenon.”

Monday marked the 20th anniversary of the publication of Harry Potter and the Philosopher’s Stone, the first installment of J.K. Rowling’s epic tale of a boy wizard, his band of friends, and their struggle to defeat a super villain named Lord Voldemort. Since “The Boy Who Lived” entered the literary world on June 26, 1997, the Potter books have sold more than 500 million copies worldwide while spawning a multi-billion-dollar film franchise. Virtually no segment of the entertainment industry has been left untouched by the series, with Potter-based theme parks, video games, stage plays, memorabilia, and fan fiction cropping up like proverbial weeds.

Kelly attributed Harry Potter’s widespread appeal to Rowling’s ability to indulge the escapist fantasies of readers of all ages, from the schoolchild to the retiree. She noted the similarities between the early life of the eponymous boy wizard and Sigmund Freud’s family romance theory, in which young children fantasize that they are the offspring of parents of higher social standing than they truly are. “Harry Potter was a much abused outsider, living in a closet under the stairs in the home of his Muggle family,” she explained, using Rowling’s term for people who lack magical ability. “Under Freud’s theory, some children fantasize that they are living with the wrong family and will be rescued, identified as a prince or princess, and then returned to their real family, which is similar to what happened in Harry Potter.” As she put it, “Why shouldn’t we have a little magic in our lives?”

Looking for the next Harry Potter

In the course description of Kelly’s class on fantasy literature, she writes, “Familiarity with the Harry Potter books and-or films assumed.” She doesn’t teach the series—the books are too long, she said—but her pupils often deliver presentations in which they analyze scenes from the movies.

Some of her students use Harry Potter’s world to make sense of their own lives. One student, she said, described the size of his room at a hostel in England as a “cupboard under the stairs,” a reference to Harry Potter’s small, dusty room in the Dursley household. “The books are older than many of the students I teach,” she noted, “but Harry Potter has shaped their world from the beginning.”

According to Kelly, Harry Potter’s success has also disrupted the publishing world, compelling literary agents to specialize in fiction for young adults. They wanted to find the next big hit—the next big YA book-to-movie-franchise. And in many cases, they have. Over the years, a raft of teen literature has received the big-screen treatment, from John Green’s The Fault in our Stars to Veronica Roth’s Divergent trilogy.

The Hunger Games trilogy—written by Suzanne Collins, published by Scholastic, and adapted into four feature films grossing nearly $3 billion—might not have existed without Harry Potter. “We probably wouldn’t have seen The Hunger Games, which I think is a phenomenon,” Kelly said. “The first book established itself in the tradition of Potter, but it was distinct enough and extremely well-written.” Other highly successful YA series’, like the Twilight saga, probably would have seen the light of day. “It was part of the vampire fad,” she said. “That would have fed it.”

Kelly thinks that a new series will eventually come along to fill Harry Potter’s shoes, giving rise to a similar pop culture phenomenon. She doesn’t know who the author will be—or what the books will be about—but she does feel pretty confident in proclaiming that fantasy will be at play. “The next thing that hits will not be entirely of this world,” said Kelly, who is currently looking for a literary agent to publish the first volume of her own post-apocalyptic trilogy. “Magic realism as mainstream literature is appealing because it’s strange, not quite of this world, and that’s what people find interesting.”